The present disclosure relates to an image forming apparatus with a plurality of optical scanning devices and particularly to a technology for suppressing a scan position shift which occurs due to a temperature difference between optical scanning devices.
Conventionally, an image forming apparatus has been known which includes an image forming unit configured to form a toner image on a surface of a photoconductive drum and provided for each of a plurality of colors. The respective image forming units are arranged along a conveying direction of a recording sheet above a conveyor belt for conveying the recording sheet and transfer toner images of the respective colors to the recording sheet conveyed in the conveying direction in a superimposing manner.
In the image forming apparatus of this type, each image forming unit deflects laser light output from a light source by a scanning lens made using optical resin with good optical properties after reflecting the laser light by a rotational polygon mirror which is driven and rotated, whereby the laser light is scanned across the surface of the photoconductive drum at a constant speed. In this way, an electrostatic latent image is formed on the photoconductive drum surface.
Here, toner images of the respective colors formed on the photoconductive drums by attaching toners to electrostatic latent images need to be transferred in a superimposing manner so as not to cause any position shift on a recording sheet. To this end, a control is executed to adjust write positions (scan positions) of the electrostatic latent images on the surfaces of the photoconductive drums. For example, a control is executed to adjust the operations of the other rotational polygon mirrors so that the rotational polygon mirror in a certain image forming unit and those in the other image forming units rotate with predetermined phase differences.
However, if the temperatures of the respective image forming units differ due to usage frequencies and arranged positions of the respective image forming units, refractive indices of the optical resins forming the scanning lenses may change to be different from each other according to temperature. This may shift laser light paths among the respective image forming units. Even in the case of executing the above control, laser light scan positions may shift among the respective image forming units.
One conventional technology is known which suppresses laser light scan position shifts occurring due to such temperature differences among respective image forming units. According to this conventional technology, when image recording is performed by operating one of a plurality of image forming units, heating means of optical scanning devices in the other image forming units are also operated so that temperature differences of the optical scanning devices in the respective image forming units fall within a predetermined range.
Further, according to another conventional technology, an image of a specific color is formed by rotating a rotational polygon mirror necessary to form the image of the specific color at a rated rotation speed and other rotational polygon mirrors at a rotation speed slower than the rated rotation speed. This enables reductions in noise, vibration, smear of the optical scanning devices and the like while suppressing scan position shifts at the time of image formation.
However, if the rotational polygon motor of one optical scanning device used for image formation is driven at a predetermined rotation speed and the rotational polygon motors of the other unused optical scanning devices are driven at a rotation speed slower than the predetermined rotation speed during single-color image formation, noise and vibration can be reduced, but temperature increase rates of the unused optical scanning devices may be reduced. As a result, it may not be possible to sufficiently reduce temperature differences among the respective optical scanning devices until the next image forming operation. This may cause scan position shifts among the respective optical scanning devices during the next image formation.
An object of the present disclosure is to reduce a possibility of causing scan position shifts among respective optical scanning devices due to temperature differences among the respective optical scanning devices without largely impairing quietness.